When the size of a solid substance is on a nm-order (nanometer-order), the specific surface area thereof is extremely large, and therefore, though it is solid, its interface with vapor or liquid is extremely large. Accordingly, its surface characteristics greatly control the properties of the solid substance. For a metal particle powder, it is known that the melting point of the powder dramatically lowers as compared with that as a bulk, and therefore, as compared with particles on a μm-order, the particle powder on a nm-order has some advantages in that it enables micropatterning in interconnecting and it may be sintered at a low temperature. Of such a metal particle powder, a silver particle powder has low electric resistance and has high weather resistance, and the cost of the metal is lower than that of other noble metals, and therefore a silver particle powder is especially expected as a next-generation interconnecting material for micropatterning in interconnecting formation.
For producing a nm-order silver particle powder, there are known a vapor-phase method and a liquid-phase method as roughly classified. The vapor-phase method is generally a method of evaporation in vapor, and Patent Reference 1 describes a method of evaporating silver under a low pressure of 0.5 Torr or so in an inert gas atmosphere of helium or the like. Regarding the liquid-phase method, Patent Reference 2 discloses a method for producing a silver colloid, which comprises reducing a silver ion with an amine in an aqueous phase and transferring the resulting silver fine particles into an organic solvent phase containing a high-molecular-weight dispersant. Patent Reference 3 describes a method for reducing a silver halide with a reducing agent (alkali metal hydroborate or ammonium hydroborate) in the presence of a thiol-type protective agent in a solvent.
Patent Reference 1: JP-A 2001-35255
Patent Reference 2: JP-A 11-319538
Patent Reference 3: JP-A 2003-253311